Phosphor-enhanced light sources are known per se and are used for substantially all kinds of light sources. Phosphor-enhanced light sources comprise a light emitter and a luminescent material. The luminescent material is arranged for converting at least part of the light emitted by the light emitter into light of a longer wavelength.
Well-known phosphor-enhanced light sources are, for example, mercury vapor discharge lamps in which the light is emitted from a discharge in which the presence of mercury vapor causes the discharge to emit ultraviolet radiation. At least a part of the ultraviolet radiation is absorbed by a luminescent material and converted into light of a longer wavelength which is subsequently emitted by the luminescent material. Such mercury vapor discharge lamp may, for example, comprise a discharge vessel in which the discharge is generated. The luminescent material is typically applied to the inner wall of the discharge vessel such that the ultraviolet radiation emitted by the discharge does not need to pass the discharge vessel but is inside the discharge vessel converted into, for example, visible light.
Alternatively, the phosphor-enhanced light source may comprise a solid-state light emitted as the light emitter. Such a solid-state light emitter may, for example, be a light emitting diode, or a laser diode, or an organic light emitting diode. The light emitted by a solid-state light emitter typically has a relatively narrow spectrum arranged around a center wavelength. The width of the spectrum may, for example, be defined by the Full Width Half Maximum (further also indicated as FWHM) of the emission peak which is a width of the emission peak measured at an intensity being half the maximum emission intensity of the light emitted by the solid-state light emitter. The FWHM of a typical emission spectrum of the solid-state light emitter is less than 30 nanometer, which is typically identified by the human eye as light of a single color. To change the color of the light emitted by the solid-state light emitter, luminescent materials may be added to generate a phosphor-enhanced light source. The luminescent material may, for example, be applied as a layer on top of the die of the solid-state light emitter, or may, for example, be dispersed in a matrix which may be located at a distance of the solid-state light emitter, a so called “remote phosphor” arrangement. The luminescent material may also be part of a mixture of different luminescent materials, for example, each generating a different color such that the mixed light, for example, generates white light having a specific color temperature. Furthermore, luminescent materials may be added to solid-state light emitters to improve the color rendering characteristics of the solid-state light emitters, as the typical emission characteristic of the luminescent materials is a relatively broad spectrum of light.
Recently new luminescent materials are being used in phosphor-enhanced light sources, such as organic luminescent materials, especially to replace known in-organic luminescent materials which are used to provide the “Red”-contribution in white light sources. Known in-organic luminescent materials which provide the “Red”-contribution have relatively poor efficiency due to their broad FWHM and therewith emission in the deep (Infra)-red. To still ensure sufficient “Red”-contribution to generate white light having the required color temperature, a relatively large amount of “Red”-contributing luminescent material is required. As such, the relatively large amount of “Red”-contributing luminescent material required results in increased costs and requires relatively high light emission intensity from the light emitter of the phosphor-enhance light source. As such, more efficient luminescent materials are required, especially to provide the “Red”-contribution to the light emitted by the phosphor-enhanced light source. As such, organic luminescent materials are introduced in phosphor-enhanced light sources which may be mixed with known luminescent materials to obtain a more efficient light converter.
Such a phosphor enhanced light source comprising organic luminescent materials in the luminescent mixture are, for example, known from the US patent application US 2006/0214578 and from the US patent application US 2006/0220531. Both cited US patent applications disclose a semiconductor light emitting apparatus which includes a packaging member, a light-emitting element mounted in the packaging member and a wavelength changer. The wavelength changer absorbs the light from the light-emitting element and emits a wavelength-converted light. The wavelength changer includes inorganic fluorescent material and organic fluorescent material.
Still, the efficiency of the light conversion of the mixture of luminescent materials as disclosed in the cited patent applications should be further improved.